1. Field of Invention
The present invention relates to arrays of micro mirrors structured by MEMS (micro electrical mechanical systems) and, in particular, to radiation detection devices using optical readout of micro mirror orientations.
2. Description of the Background Art
In recent years, MEMS devices micromachined from silicon by electronics-industry techniques, have become important as transducers of various kinds. MEMS devices may be structured individually or in arrays. When a physical effect causes a microscopic deflection of a MEMS structure having a micro mirror, it is advantageous to detect the deflection with a reflected light beam. One well known example is the atomic force microscope, whereby the position of a single microscopic probe tip is sensed optically by reflecting a laser beam from the probe. In another example, a rectangular array of a plurality of MEMS micro mirrors may have individual deflections which relate to the variations in, temperature across the MEMS array. The deflections of all the individual micro mirrors can be read out as an image by a broad light beam. In all such cases, the ultimate sensitivity and dynamic range of an instrument whose transduction mechanism is the optical readout of an array of MEMS micro mirrors is at issue.
The prior state of the art discloses a plurality of thermal detector systems based on optical MEMS, or a mechanical-optical transducer, that include micro-mirror arrays. For example, U.S. Pat. No. 6,339,219 “Radiation imaging device and radiation detector,” discloses an imaging device comprising a substrate transmissive to infrared radiation, an infrared lens system, a conversion unit for converting infrared radiation into displacements, and a readout optical system. Similar devices are disclosed in U.S. Pat. No. 6,469,301, “Radiation detectors including thermal-type displaceable element with increased responsiveness,” and U.S. Pat. No. 6,835,932 “Thermal displacement element and radiation detector using the element.”
FIG. 1 shows a conventional imaging device 10 comprising a visible-wavelength imager, such as charge coupled device (CCD) 11, to read-out the converted image, and a visible light source 13 to provide readout light. A first lens system 15 is used to guide readout light from the visible light source 13 to an array of micro mirrors 17 of a MEMS device 19. A ray flux limiting part 21 is used to selectively pass only desired ray fluxes transmitted in a portion of readout light reflected by the plurality of pixels after passing through the first lens system 15 and an aperture 23. A second lens system 25 forms positions conjugate with the plurality of reflection parts in conjunction with the first lens system 15. The photosensitive surface of the CCD 11 is placed at the conjugate positions.
In operation, the surface of the MEMS device 19 provides a uniform reflection when there is no physical disturbance to any of the micro mirrors 17. In certain applications, such as infrared imaging, an objective lens 29 is provided to project an infrared image onto the MEMS device 19. When such infrared radiation is incident, each corresponding micro mirror 17 may tilt and deflect readout light away from the aperture 23, thus modulating light transmitted to the CCD 11 in proportion to the incoming infrared radiation level. This design effectively converts infrared induced micro bending of the micro mirrors into intensity change at a visible read-out illumination. The optical readout from the CCD 11 typically tracks deflections of all micro mirrors 17 in the MEMS device 19 and provides an intensity map as an output.
Certain applications, such as imaging systems, could benefit from an imaging device having dynamic temperature ranges larger than that achievable using the prior art designs described above. It is appreciated that, as the device responsiveness increases, the corresponding readout dynamic range is proportionately reduced. This can be qualitatively described by Equation 1, where the temperature dynamic range (δTt)max is inversely proportional to the responsivity of the sensing device:(δTt)max=100%/Responsivity  (1)
What is needed is a mechanical-optical transducer that provides for a greater dynamic range than imaging devices in the prior state of the art, but where the sensitivity of the mechanical-optical transducer is not correspondingly decreased.